This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2001-331470, the content of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a field emission-type electron source including an electron source element for emitting electron beams by means of the field emission phenomenon and a method of biasing such a field emission-type electron source.
2. Description of the Prior Art
Heretofore, there has been known a field emission-type electron source (hereinafter referred to as xe2x80x9celectron sourcexe2x80x9d) including a lower electrode, a surface electrode (upper electrode) composed of a metal thin-film opposed to the lower electrode, and an electron transit layer interposed between the lower and surface electrodes. In this kind of electron source, when a certain voltage is applied between the lower and surface electrodes such that the surface electrode has a higher potential than that of the lower electrode, a resulting electric field between the electrodes induces the flow of electrons from the lower electrode to the surface electrode through the electron transit layer. After passing through the electron transit layer, the electrons are emitted through the surface electrode.
A conventional electron transit layer for use in this kind of electron source includes a strong-field drift layer (hereinafter referred to as xe2x80x9cdrift layerxe2x80x9d) composed of an oxidized or nitrided porous polycrystalline silicon layer (see Japanese Patent Publication No. 2987140, as an example). There have also been known an electron source using an oxidized or nitrided monocrystalline silicon layer as the electron transit layer, and a MIM (Metal-Insulator-Metal) type electron source using an insulator layer as the electron transit layer (see Japanese Patent Laid-Open Publication No. 7-226146, as an example).
FIG. 15 shows one example of conventional electron sources having a drift layer. Referring to FIG. 15, an electron source 10 includes a drift layer 6 which is composed of an oxidized porous polycrystalline silicon layer (polycrystalline silicon layer transformed into a porous structure) and formed on a front surface of an n-type silicon substrate 1 as a conductive substrate through a non-doped polycrystalline silicon layer 3. A surface electrode 7 composed of a metal thin-film (e.g. gold film) is formed on the drift layer 6. An ohmic electrode 2 is formed on a back surface of the n-type silicon substrate 1. The n-type silicon substrate 1 and the ohmic electrode 2 make up a lower electrode 12. There has also been proposed an alternative electron source having the drift layer 6 formed directly on the front surface of the n-type silicon substrate 1 without interposing the polycrystalline silicon layer 3 between the n-type silicon substrate 1 and the drift layer 6.
While the lower electrode 12 of the electron source 10 in FIG. 15 is made up of the n-type silicon substrate 1 and the ohmic electrode 2, an alternative electron source 10 has been proposed in which a lower electrode 12 of a metal material is formed on a front surface of an insulative substrate 11 composed, for example, of a glass substrate, as shown in FIG. 16.
The electron source 10 in FIG. 15 or 16 is operable to emit electrons through the following process. A collector electrode 21 is first positioned opposed to the surface electrode 7. Then, a DC voltage Vps is applied between the surface electrode 7 and the lower electrode 12 such that the surface electrode 7 has a higher potential than that of the lower electrode 12, while forming a vacuum space between the surface electrode 7 and the collector electrode 21. Additionally, a DC voltage Vc is applied between the collector electrode 21 and the surface electrode 7 such that the collector electrode 21 has a higher potential than that of the surface electrode 7. By appropriately setting the respective DC voltages Vps, Vc, electrons injected from the lower electrode 12 are drifted across the drift layer 6, and then emitted through the surface electrode 7. The one-dot chain lines in FIG. 15 or 16 indicate the flow of the electrons exe2x88x92 emitted through the surface electrode 7. The electrons reaching a front surface of the drift layer 6 can be considered as hot electrons. Thus, such electrons readily tunnel through the surface electrode 7 and emitted into the vacuum space.
Terms xe2x80x9cdiode current Ipsxe2x80x9d and xe2x80x9cemission current (emission electron current) Iexe2x80x9d as used in the electron sources 10 generally mean a current flowing between the surface electrode 7 and the lower electrode 12 and a current flowing between the collector electrode 21 and the surface electrode 7, respectively. In the electron sources 10, a greater ratio of the emission current Ie to the diode current (Ie/Ips) provides higher electrode emitting efficiency. The above electron sources 10 is operable to emit electrons even if the DC voltage Vps to be applied between the surface electrode 7 and the lower electrode 12 is set in a low range of about 10 to 20 V, and the emission current Ie is increased as the DC voltage Vps is increased.
For example, the electron source 10 as shown in FIG. 15 or 16 is applicable to an electron source for displays (see FIG. 12).
In the conventional electron sources 10, the drift layer 6 includes traps acting to capture electrons. Thus, some of electrons injected from the lower electrode 12 into the drift layer 6 are captured by the traps, which will reduce the diode current Ips and the emission current Ie with time, resulting in relatively short lifetime of the electron sources.
In this context, there has been proposed an electron-source biasing method in which an electric field having a polarity to be alternately inversed is applied between a lower electrode and a surface electrode to allow captured electrons in traps to be released and emitted (see the Japanese Patent Laid-Open Publication No. 7-226146, as an example). This electron source is a MIM type electron source including an upper electrode (the surface electrode) made of metal or highly-doped semiconductor, the lower electrode made of metal or highly-doped semiconductor, and an insulator layer interposed between the upper and lower electrodes. This electron source is operable to alternately inverse the polarity of a voltage to be applied between the upper and lower electrodes, so that some of electrons to be captured by a first trap formed in the insulator layer adjacent to the upper electrode and a second trap formed in the insulator layer adjacent to the lower electrode are moved between the first and second traps to facilitate effective emission of the electrons.
However, assuming that the biasing method disclosed in the Japanese Patent Laid-Open Publication No. 7-226146 is applied to the electron source 10 as shown in FIG. 15 or 16, even if an electron captured by a trap in the drift layer 6 is released from the trap, the released electron will be captured by another trap in the drift layer 6. Thus, the diode current Ips and the emission current Ie will be undesirably reduced with time, and thereby adequate lifetime cannot be obtained.
Japanese Patent Laid-Open Publication No. 11-95716 discloses an electron source biasing method used in an image display device having the electron source elements in a matrix arrangement, in which after a scanning operation in each frame period, a reverse voltage is applied simultaneously to all of the electron source elements to allow captured electrons in traps to be released and emitted. This biasing method has the following problems.
(1) Regardless of whether a voltage has been applied to bias (or actuate) each of the electron source elements, the reverse bias voltage is applied simultaneously to all of the electron source elements in each frame period. Thus, a significant fluctuation will occur in respective electron emission characteristics of the electron source elements. This fluctuation cannot be controlled because the bias condition of each of the electron source elements is dependent on an image to be displayed thereby.
(2) The reverse bias voltage is applied simultaneously to one electron source element which has been biased in the initial stage of one frame period and another electron source element which has been biased in the later stage of the frame period. Thus, these electron source elements will have a difference in a waiting time between the completion of the bias and the initiation of the application of the reverse bias voltage, and consequently a significant fluctuation will occur in respective electron emission characteristics of the electron source elements. This fluctuation is increased as the use period of the electron source elements gets longer because the scanning sequence of the electron source elements is fixed.
In view of the above problems of the conventional electron sources, it is therefore an object of the present invention to provide an electron-source biasing method capable of providing a longer lifetime of an electron source.
It is another object of the present invention to provide an electron source having a longer lifetime.
In order to achieve the above objects, the present invention provides a method of biasing an electron source (field emission-type electron source). The electron source used in this biasing method includes an electron source element having a lower electrode, a surface electrode, and a drift layer (strong-field drift layer) interposed between the lower and surface electrodes. When a forward voltage is applied between the lower and surface electrodes such that the surface electrode has a higher potential than that of the lower electrode, a resultingly induced electric field allows electrons to pass through the drift layer. The electrons passing through the drift layer is emitted through the surface electrode. The electron source biasing method includes the steps of applying a reverse voltage (negative voltage, reverse bias voltage) the electron source element after the forward voltage (positive voltage, forward-bias voltage) has been applied to the electron source element, and applying no reverse voltage to the electron source element after the forward voltage has not been applied to the electron source element.
According to the above electron source biasing method, the reverse voltage is applied to the electron source element only if the forward voltage has been applied to the electron source element. This makes it possible to suppress the fluctuation of electron emission characteristics caused by different bias conditions of the electron source element. This effect can be enhanced by controlling the reverse voltage in response to the absolute value of the forward voltage. The effect of applying the reverse-bias voltage can be constant by setting the time-period of applying the forward and reverse voltages at a given or constant value, which allows the fluctuation of electron emission characteristics to be more effectively suppressed. Further, it can be avoided to apply unnecessary voltage to the electron source element because no reverse voltage is applied when no forward voltage has been applied. This provides enhanced reliability of the electron source element. In a device having a matrix arrangement such as displays, electrons can be emitted from an electron-emitting surface with enhanced uniformity.
The present invention provides another method of biasing a field emission-type electron source including a plurality of electron source elements. A drift layer in each of the electron source elements includes a number of nano-order semiconductor nanocrystals, and an insulating film formed on the surface of each of the semiconductor nanocrystals. The insulating film has a film thickness less than the grain size of each of the semiconductor nanocrystals. In this electron source biasing method, a reverse voltage is applied simultaneously to all of the electron source elements in each frame period.
The present invention further provides another electron source including an electron source element and a bias circuit. The electron source element includes a lower electrode, a surface electrode, and a drift layer interposed between the lower and surface electrodes. When a forward voltage is applied between the lower and surface electrodes such that the surface electrode has a higher potential than that of the lower electrode, a resultingly induced electric field allows electrons to pass through the drift layer. The electrons passing through the drift layer are emitted through the surface electrode. The bias circuit includes a forward voltage applying circuit and a reverse voltage applying circuit. The bias circuit is operable to apply a reverse voltage to the electron source element through the reverse voltage applying circuit after a forward voltage has been applied to the electron source element through the forward voltage applying circuit, and to apply no reverse voltage to the electron source element through the reverse voltage applying circuit after the forward voltage has not been applied to the electron source element through the forward voltage applying circuit.
In this manner, the present invention provides extended lifetime of the electron source.